The present invention relates to a method of operating a vehicle having an electric powertrain that includes a high voltage bus, a traction inverter and an electric machine, with the traction inverter being coupled between the high voltage bus and the electric machine. The method includes generating an electric voltage for the high voltage bus with the electric machine and the traction inverter when the electric machine is driven mechanically. Furthermore, the present invention relates to an electric power train for a vehicle.
It is common for an electric motor to be directly connected to the output through a defined fixed gear ratio in pure electric drive trains. This lowers costs and increases reliability by not having a clutch within the transmission. In many vehicle conditions this approach is sufficient. However, there are conditions in which the lack of a clutch introduces undesired consequences. One such event is during vehicle dinghy tow. When the vehicle is towed, the electric machine increases in speed proportion to the road speed and the gear ratio since the electric machine is directly connected to the output powertrain. In electric machines with permanent magnets (PM) such as Interior Permanent Magnet (IPM) machines, the rotation of the magnets creates a back electromotive force (emf) voltage. The range of the back emf voltage is dependent on the design of the PM machine. This back emf poses a problem for two major reasons:                1) regulations generally require a maximum high voltage (HV) bus voltage of 60 V during an uncontrolled state; and        2) the power electronics have voltage limitations that must be met or permanent damage is possible, while designing the PM machine to limit the back emf too much would prevent the capability of regenerative breaking.        
Generally during a tow event, the 12 V system is disabled and thus the power inverters are disabled, preventing an active control method to limit the voltage. Possible solutions would be to apply a continuous 3-phase short. However, temperatures of the power electronics and electric machine quickly reach their limitations without proper cooling that is generally based on the 12 V system. Cooling systems are dependent upon having an active 12 V system or costly HV cooling pumps. Even HV cooling pumps would have difficulties to function properly over the complete voltage range of the back emf of the electric Machine (which could be in a range, for example, of from 60 V to 400 V).
U.S. Pat. No. 6,866,350 describes regenerative breaking in an electric vehicle when towed. The regenerative breaking is actuated to recharge the battery and break the vehicle. When the driver of the towing vehicle depresses the brakes of the vehicle, the electrical vehicle applies regenerative breaking in proportion to a breaking signal from the towing vehicle. This requires an external electrical connector between both vehicles. The issue of providing cooling for the inverters and electric machines is not addressed.
Furthermore, U.S. Pat. No. 6,793,034 discloses a method to disconnect drive wheels from the powertrain of any electrically powered vehicle. A vehicle controller monitors input from, for example, an inertia switch and electric motor generator conditions and can disconnect the output shaft from the drive wheels in a predetermined vehicle condition such as during a rear-end-collision. This requires a mechanical mechanism within the transmission to provide the disconnect, along with a microprocessor to provide the proper disconnect based on defined criteria. Such assembly is similar to having a clutch, which is expensive.
A system and method for powering accessories in a hybrid vehicle are known from U.S. Pat. No. 7,391,129. For this purpose an electric accessory motor is used which is mechanically connected to the combustion motor to provide the mechanical movement. In other words, there is provided an alternator with an ultra-capacitor to supply power when the combustion engine is off during “stop/start” events. However, such system does not work for purely electric vehicles.
Moreover, German patent document DE 10 2008 004 300 A1 discloses a vehicle including a generatively operable electric machine coupled to a wheel, and means for detecting whether the vehicle is towed. During towing the electric machine is operated to supply the on-board network of the vehicle with power. Specifically, a converter is provided to convert a high voltage provided by the electric machine into low voltage.
One object of the present invention is to provide a cost effective solution which allows for towing of an electric vehicle without a clutch.
According to the present invention there is provided a method of operating a vehicle having an electric powertrain, the powertrain comprising a high voltage bus, a traction inverter and an electric machine, wherein the traction inverter is coupled in between the high voltage bus and the electric machine, the method including the steps of:                generating an electric voltage for the high voltage bus with the electric machine and the traction inverter when the electric machine is driven mechanically;        limiting the generated electric voltage;        converting the limited voltage to a predetermined voltage lower than the limited voltage; and        powering a unit of the vehicle with said predetermined voltage.        
Furthermore, there is provided an electric powertrain for a vehicle comprising                a high voltage bus;        an electric machine; and        a traction inverter coupled in between the high voltage bus and the electric machine and adapted for supplying an electric voltage to the high voltage bus when the electric machine is driven mechanically,wherein        the traction inverter is adapted to limit the generated electric voltage,        converting means convert the limited voltage to a predetermined voltage lower than the limited voltage, and        a unit attached to the electric powertrain is powered with said predetermined voltage.        
The advantage of the present invention is that an electric vehicle can be continuously towed at any speed without an expensive and low reliable mechanical clutch inside the transmission. If the electric voltage generated by the electric machine is limited, the current vehicle architecture can be used with only an addition of a small dc/dc converter box. A costly HV cooling pump is not required. The vehicle HV bus can be maintained to a desired programmable limit based on either regulations or component safety.
In a preferred embodiment the generated electric voltage is limited to a maximum lower than 100 V dc. Such a limitation of the generated voltage allows the application of a small, low-cost dc/dc converter.
Preferably the generated electric voltage varies like a sawtooth between two pre-given voltage levels. Such a sawtooth voltage can be easily obtained by the traction inverter when controlled with respect to a maximum level. Specifically, for an electric machine designed as 3-phase motor and for a corresponding traction inverter including six switches (usually realized by a transistor), two for each phase, the limitation of the voltage may be effectively obtained by operating the traction inverter in two states alternatingly: The first state in which all six switches are open so that the electrical machine is disconnected from the high voltage bus, and a second state in which three of the six switches are closed so that the three phases of the motor are shorted. The internal capacity of the HV bus will keep the voltage output from the traction inverter in the generator mode below a certain limit, when the traction inverter is switched between the first state and the second state.
According to a further aspect of the invention, the frequency of the sawtooth may be determined by the discharge rate of the high voltage bus being disconnected from the electric machine. Preferably the frequency of the sawtooth is less than 3 kHz and specifically less than 500 Hz. The discharge may be controlled by a dc/dc converter properly. If the switching frequency of the traction inverter is reduced, for example, from 10 kHz to 300 Hz, the electrical losses of the system may be halved.
In a preferred embodiment the unit attached to the electric powertrain and powered by the converted voltage is a cooling pump for cooling at least the electric machine for the traction inverter. Thus, the electric machine and the traction inverter are not heated excessively during a towing situation.
The lower voltage which is obtained when converting the limited voltage may be a 12 V dc voltage. Thus, although other devices of the low volt bus system of the vehicle can be supplied with sufficient energy when the vehicle is towed.
The above-described electric powertrain is preferably implemented in a motor vehicle like an electric car. However, it also may be implemented in any other electrically driven vehicle in a rail bound system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.